Method of carbonating lime



April 30, 1940. E. STUMP 2,198,640

METHOD OF CARBONATING LIME Fil'ed Nov. 27, 1936 CflNcENTRn r/a/v METERW'TNESS TIME INVENTOR YCZM MW I W y- #0 3465 [.fifl/MP ATTO NEYSPatented A r. 30, 1940 UNITED STATES PATENT ,OFFICEV 3 Claims.

My invention relates to the carbonation of aqueous suspensions ofcalcium hydroxide obtained by the burning and slaking of calcite or'dolomite or in any other manner, and particularly of suspensionscomprising a liquid phase represented by a more or less saturatedsolution of calcium hydroxide and a solid phase of suspended particlesof undissolved base, commonly known as lime milk or milk of lime.

It is an object of the present invention to provide a method ofcarbonating aqueous suspensions of calcium hydroxide whereby any one ofa number of different kinds of precipitated chalk,

each of more or less uniform character, may be obtained'at will, andparticularly a precipitated chalk of unusual density and of largeparticle size which is easily separated by settling, centrifuging orfiltering.

This process has utility in and of itself in producing a comparativelyuniform chalk of selected characteristics and especially a dense chalkwhich I is suitable for use, for example, in connection chalk.

with the manufacture of paint pigments and pa per. However, it hasafurther utility, and this I regard as the greater utility," ofpermitting a separation of calcium hydroxide and magnesium oxide (andhydroxide) in such a manner as to .make it possible to obtain the lattersubstance more cheaply in relatively pure condition from its usualsource, namely a mineral containing both calcium andmagnesium, forinstance, dolomite.

It is accordingly a further object of the invention to provide a methodof treating aqueous S1187 pensions of mixtures of calcium hydroxide andmagnesium oxide (and generally also magnesium hydroxide) obtained, forexample, by the'slaking of limes produced by the burning of dolomite orother minerals containing calcium and mag-, nesium carbonates,-wherebyan efllcient separation of .the calcium from the magnesium is secured ina simple and inexpensive manner by the precipitation of the calcium inthe form of dense particles of calcium carbonate, the magnesia re--maining in a more or less colloidal form which can readily be separatedfrom the precipitated Another object of the invention is further totreat the magnesia suspension so separated in a manner which willfacilitateits filtering or precipitation preparatory to its obtainmentinthe dry condition. I

, Other objects and advantages of the invention will appear from thefollowing more detailed description of my improved process. ,l

Briefly defined, the present invention involves predetermining thecharacter ofthc chalk precipitate by continuous carbonation of a slakedcalcitic or dolomitic lime milk at definitely maintained calciumhydroxide concentrations. I have 5 found that the natureof the calciumcarbonate precipitate, and especially its density and particle size, andhence the ease withwhich it can be separated gravimetrically from othersuspended matter, is a function of the concentration of the 10 calciumhydroxide at the moment of reaction with the carbon ,dioxide, and to acertain extent of other factors, such as temperature. Inaccordance withthe invention, therefore, the character of the CaC0a, precipitateisptedetermined 16 and controlled bymaintaining the concentration ofCa(OH)'z at amore or less fixed value during the precipitation of thethe CaCOa, or within a limited rangeof values, by conducting thecarbonation continuously and regulating the relative to feeds of limemilk and carbon dioxide.

In the preferred manner of carrying "out the inventiomthe calciumcarbonate is continuously precipitated out of a solution of calciumhydroxide which is maintained continuously in a state of 88 unsaturationwith respect to the hydroxide. This may be accomplished by conducting astream of lime milk, containing the excess Ca(OI-I) a in the iormoi'flne'particles. into a relatively large body of water having a lowconcentration (say,

only'10-15% ofsaturation) of calcium hydroxide, whereupon the suspended,solid hydroxide immediately dissolves. The solution is reactedcontinuously with CO2, which is fed at a rate equivalent to that of thehydroxide, the continuous preelicitation of 09.603 leaving the solutionpermanently unsaturated with respect to the hydroxide. The carbonateremains in suspension and is preferably continuously withdrawn, the

emuent suspension containing an' amount of un- 14b:

reacted .Ca(OH)z in solution correspondingi approximately to theconcentration of the Ca(0I-I) 2 in the unsaturated solution beingcarbonated. The particles or crystals of C aCO obtainedunder theconditions stated are relatively dense 4B and'settle readily. Themagnesia present in the treated lime milk remains in-suspension, separaetion of the calcium from the magnesium being thus readily efl'ected.Suitable methods for main- C5(0H so taining' the desired concentrationof will be described below. f v

3 It will be evidentthat by the continuous process lust described, theparticles of 011.00:

remain suspended in a solution of Ca (OH) 2 whose cone ntrationissubstantlally the'same as that u if the latter is times the volume oflime milk charged into the-vessel per minute, then, since the outflow isthe same as the inflow, the average length of time that a particle ofCaCOa will remain in the vessel is 15 minutes. It is my belief thatduring this period the small particles of CaCOa act as nuclei upon whichfurther quantities of 'CaCOa of the same-physical nature deposit, thusproducing relatively large and dense particles which separate morereadily from the colloidal magnesia.

Investigations conducted by me on the character of the chalk precipitateobtained by the batch carbonation of lime milk as the concentration ofcalcium hydroxide progressively diminishes have shown that the productprecipitated out of solution in the early stages of such batchcarbonation, that is, from milk of lime having a large amount ofsuspended excess Ca(OH)2, is of a relatively soft consistency and isdiflicult to handle, and for many purposes is undesirable in thiscondition. This product is even less desirable when it is sought toseparate calcium from magnesium, for it does not settle quickly andcarries down with it large proportions of magnesia, and so makes anefficient separation based on diflerences in density extremelydifiicult, if"

not impossible. 1

On the other hand, the continuance of the batch carbonation to the pointwhere calcium bicarbonate is formed by reaction of excess CO2 withCaCOs, is objectionable because the solution must subsequently bc heatedto convert the bicarbonate to the normal carbonate. Where it is desiredto separate calcium from magnesium, such overcarbonation is undesirablealso for the reason that a part of the magnesium goes'into solution,while the calcium bicarbonate that is formed tends to precipitateunconverted. magnesia with it,. so-that eflicient separation of the'calciumfrom' the magnesium by gravity or centri- ,fuging is'renderedpractically'impossible, since only the relatively small proportion ofmagnesia that has been converted to the soluble bicarbonate is separatedfrom the mixture of precipitated calcium and magnesium.-

The invention accordingly contemplates carrying outthe carbonationcontinuously under predetermined conditions of reduced calcium hydroxideconcentration (the concentration being, however, greater than zero),such that the production of diificultly separable forms of precipi-'tate is avoided. This I accomplish by maintaining the concentration ofcalcium hydroxide (dissolved .and suspended, if undissolved hydroxide.

continuously at a value lower than is present) .that corresponding tothe mid-point of a batch carbonation of a quantity of the same originalsuspension.

Various methods may be employed for maintaining the concentration ofCa(OH)z at. the value required to produce the desired precipitate duringthe continuous carbonation of the lime solution. Thus the regulation ofthe relative feed of CO2 and lime milk may be'based on the retinuouslyreplaced sults of the titration with. acid of samples peri odicallytaken from the eflluent material, the

. being carbonated; This mode of control is excellcntly suited for theregulation of an unsaturated solution and has the important advantagethat it caneasily be made automatic, so as to cause the production of asubstantially uniform precipitate, particularly when the temperature ofcarbonation is kept relatively constant. Suitable apparatus formaintaining the liquid being carbonated unsaturated with respect to.

Ca(OH)2 will be described below.

The invention will be further described by relerence to the accompanyingdrawing wherein.

. Fig. 1 illustrates diagrammatically a suitable 'apparatus'for carryingout the invention; and

Fig. 2 shows a time-resistance curve of the batch carbonation of asuspension of calcium hydroxide (milk of lime) at 40 C.

The curve in Fig. 2 shows the change in the resistance of an aqueoussuspension of calcium hydroxide containing floating solid particles, and

originally saturated in the liquid phase, asthe carbonation thereofproceeds by r 'thebatch method. The abscissae represent time, and alsoamount of, Ca(OH)z (solid represent the total Thecurve is for atemperaand liquid phases).

ture of 40 0., this temperature being maintained constant bythermostatic control. As the carbonation proceeds, the resistance of thesolution phase remainsapproximately constant from the point A to thepoint B. This is due to the fact that as dissolved Ca(OI-Dz continues tobe precipitated in the form of the carbonate, it is conby the solutionof solid, suspended Ca(OH) 2. At the point B, the suspended solidCa(OH)z is exhausted, and from then on the concentration of the liquidphase continuous'ly falls and the resistance increases. As thecarbonation proceeds with the accompanying fall of concentration ofelectrolyte, the resistance V begins to increase rapidly to the point Cand from such point it rises still more rapidly until the point D isreached, which theoretically corresponds to a 'zero concentration ofC'a(OH)2 and to the solubility of CaCOa and of other substances present.This point D may be termed the isoelectric point. If the carbonation iscontinued, the resistance falls quite rapidly to approximately the pointE from which point on it remains prac-' ticaliy constant. I As explainedabove, the fall in resistance from D to E- is probably 'due' to theformation of calcium bicarbonate.

The interval from B to D represents the range of unsaturation of thecalcium hydroxide solution. In accordance with the preferred mode ofcarrying out the invention, the carbonation of milk of lime is conductedcontinuously and the to 15% of saturation,'- so that small changes inconcentration will be readily ascertainable in suspension. This appearsto be due to the pres-. ence of excess solid calcium hydroxide duringthe carbonationand it is my theory, although I do view of the rapid rateof change of resistance with respect to change in concentration at'suchstrengths. Somewhat lower concentrations than 10-15% may, however, beemployed while yet realizing the advantages of.. my invention and alsohigher concentrations, as explained hereinabove.

The presence of MgO and Mg(OH), as in a dolomitic lime, does notmaterially change this cium and magnesium hydroxides, the separation ofthe calcium from the magnesium is made very difficult. This is due tothe fact that when the treatment 18 011 the carbon dioxide side of-theisoelectric point, the excess CO2 converts the carbonate to' the moresoluble bicarbonate and the particles of calcium carbonate ultimatelyobtained from the latter are not as hard and as .dense as the productobtained on the calcium hydroxide side; while in the treatment ofdolomitic lime milk, the magnesia suspension. tends to be carried downwith the calcium carbonate,

' all of which is prejudicial to efficient separation.

Another reason for avoiding passing the 150-, I

electric point is that the magne'sia'is coagulated beyond such point andhence is carried down with the calcium carbonate, thereby reducing themagnesia recovery. A gravity separation, or a sepfrom the magnesia willunder such circumstances not be as eflicient as in the case of a mixturewhich has been maintained between the points F and G on the lime side.

The resistance curve shown in Fig. 2 applies, already indicated, tobatch carbonation at about 40 C. or higher. It does not, of course,represent the conditions existing in my improved process and is employedonly to indicate the Ca(OH) 2 concentration for any found resistance. Atlower temperatures, say 20 0., there appears clearly also a false orsecondary isoelectric point at an earlier stage in a batch carbonationthan the true isoelectric point. This secondary isoelectric pointrepresents an intermediate high resistance condition in the solutionwhen the latter should still contain a large quantity of Ca(H)2. Uponfurther carbonation, the resistance fallsir apidly to approximately theoriginal value, after which it again rises to the true isoelectric pointafter all of the calcium hydroxide has gone into solution and has beenprecipitated as carbonate. This secondary isoelectric point correspondsto the point H in Fig. 2 and is of dlflerent height (resistance) atdifierent temperatures and at different rates of feed of CO Itdoes' not,however, seem to occur at about 40 C. or higher.

The precipitate obtained by the carbonation (batch process) at thislower temperature (20 C.) of a commercial milk of lime up toapproximately the secondary isoelectric point appears to -consist,inlarge part, if not entirely, of relatively light particles 'which'do notsettlereadily, and in the'case of the carbonation of a slaked dolomiticlime are diflicult to separate from the magnesia not wish to beunderstood as being committed thereto, that under these conditions, abasic carbonate, probably Ca2(OH)zCOa, isflrst formed which islater'broken up by additional carbon dioxide into calcium carbonate,this transition being responsible for the softness of the particles ofthe chalk. The product precipitated in advance oi the secondaryisoelectric point, and of the point H in Fig; 2, and in the region ofsuch point, is generally of less desirable quality, at least whenseparation from magnesia is desired. This secondary isoelectric point,like the point H above to separate the calcium from the magnesium or,

in general, to increase the relative MgO content of a calcium-magnesiumlime.

I accordingly exclude from the scope of my claims the carbonation of asuspension which is continuously maintained at a Ca(OH)a concentration(solid and dissolved) ranging from that of the original suspension tothat of the midpoint of the carbonation, the process according to theinvention being conducted under such conditions that the concentrationof total Ca(OH)2 aration by a centrifuge, of the precipitated chalk(that is, both dissolved and suspended hydroxide) is maintainedapproximately constant at a value greater than zero. (thetrueisoelectric point) butsmaller than that corresponding tothe secondaryisoelectric point or the equi'molecular condition of CMOH): and CaCOa.This range of concentrations, of course, includes the'permanentlyunabove discussed.

Theapparatus shown schematically in Fig; 1'

saturated condition of the .C3.(OH)2 solution comprises a tank III ofstainless'steel or any other suitable material which is provided with anagitator shown in the form of a paddle wheel ll hydroxide (slakeddolomitic lime), or other calcium hydroxide-containing suspension, isintroduced, preferably by means of one or more spray nozzles l3 whichdisperse the liquid into a large number of fine streams. into the carbondioxideladen atmosphere within thetank. The nozzles l3 pass throughsuitable openings in the cover plate I which is fastened in any suitablemannor, as by threaded studs and nuts, shown at l5, to horizontalflanges l6 of the tank, a gasket I1 being interposed to insuregasitightness. The carbon dioxide inlet is shown at l8 and the gasoutlet at l9. Any suitable trap (not shown) may be provided in theoutlet to prevent escape of CO2. A baflle plate 20 may be employed todirect the carbon dioxide upon the rotating paddle wheel.

The paddle wheel stirs the liquid into a violent cium hydroxide andmagnesium oxide and I froth, producing intimate contact between thesolution and the gas over large areas. The gas may be that obtained bythe burning of calcite or dolomite, flue gases or any other gas rich incarbon dioxide.

For maintaining the solution of calcium hydroxide at the selected degreeof unsaturation, I provide an electrode 2| which passes through asuitably insulated stufiing box 22 and is connected by a conductor 23with a mechanism for measur-' ing concentration, as by way ofconductivity, re- I sistance, pH concentration; or in any other suitablemanner. Where the tank is made of metal, it may form the other electrodeand can be connected by a conductor 25 to the meter. If thetank is madeofnon-conducting material, two

electrodes 2| will beempioyed and supported at a suitable distance fromeach other.

It-will be understood that no particular size or arrangement ofelectrodes is necessary for the termination of theconcentration. Theelectrodes may be of any suitable size and they may be spaced by anydesired distance. Where the concentration is to be maintained bymeasurements of resistance or conductivity, the resistance orconductivity for a given arrangement of electrodes at the temperature ofoperation may be 7 determined for different concentrations of calciumhydroxide and a curve plotted, so that the concentration of dissolvedbase can be readily read off for any found resistance. During thecarbonation the resistance is maintained at the desired or zero valuecorresponding to the selected degree of unsaturation at which the liquidbeing treated is to be maintained. Variations from this zerof value ateither side, should be limited'to such ranges that the solution neitherreaches the isoelectric point, nor yet becomes so highly concentratedthat the character of the chalk precipitate is substantially altered.The measuring instrument 24 may form part of or be connected to acontrol device, such as a damper or other valve device, forregulatingthe feed of carbon dioxide and lime suspension relatively to each otherin such a manner that the desired optimum concentration is maintained. x

It will be understood that the apparatus hereinabove described isequally adapted for the carbonation of ordinary milk of lime suspensionand of a suspension of slaked dolomitic lime. The carbonation maycontinue indefinitely so long as the gas and lime milk are fed at suchrates that the resistance or concentration of calcium hydroxide insolution is held at the proper point. Where a suspension of calciumhydroxide is em-. ployed, I prefer to separate the chalk by filtering;in the case of the treatment of dolomitic lime, I prefer to effectseparation of the chalk from the suspended magnesia by gravityprecipitation'or by centrifuging.

To avoid local over carbonation, resulting in local passing, even ifonly. momentarily, of the isoelectric point D with resulting loss "ofmagnesia to the calcium carbonate sludge, I prefer cial scale, I haveemployed milk of lime containing from 4% to 8% of calcium hydroxide insuspension. As the mixture was introduced into the tank, the paddlewheel I I rotating, as aforesaid, at approximately 400- R.-P. M., thesuspended calcium hydroxide dissolved almost immediately, due to themargin of unsaturation of the relatively large body of liquid in thetank,

and was immediately thrown out by practically instantaneous reactionbetween the dissolved calcium hydroxide and the carbon dioxide whichkept the liquid phase unsaturated with respect to calcium hydroxide. Theelectrodes were of such size and so spaced that the resistance 01' themilk of lime solution (saturated) was- 30 ohms. The feed of milkof limeand of carbon dioxide was. so adjusted that during operation theresistance was between 100 and 200 ohms.

As it is desirable to avoid precipitation of cal- I cium carbonate whilethe suspension contains an amount of Ca(OH)2 corresponding approximate-I 1y to the mid-point H of the carbonation, or even greater amounts, andas it is desirable also to avoid operating-on the carbon dioxide side ofthe isoelectric point, the process may advane tageously be begun byfirstintroducing water into the apparatus while the gas is shut oiT. The milkof lime is then-fed into the carbonator,

' the resistance between the electrodes being carefully observed. As theconcentration of calcium hydroxide solution increases, the resistancefalls. Whenenough lime has been fed in to bring the resistance to about150 ohms in the commercial apparatus just described (that is, within therange F--G, Fig. 1), the gas valve is opened and the carbonation begun.The feed of the reacting materials is so controlled that the resistancecorresponding to the range F--G is continuously maintained. Obviously,automatic apparatus can be employed to open the gas valve as soon as thepredetermined resistance [is reached and the! proper feed of lime milkand gas regulated automatically.

I may also start with the carbonator fullof a previously made lot ofcalcium carbonate suspension which has not been brought. past theisoelectric point and has a concentration of Ca(OH)z lying between the,points H and-D in"- Fig.2, or preferably between F and G.

The outflowing suspension will, ,of course, contain approximately thesame concentration of Ca(OH)z as that in the" carbonator. This amountsto about 1% of total solids. For many uses, as for the manufacture ofmagnesia brick or lagging-this small amount of calcium hydroxide isunobjectionable. ,Where, however, a higher degree of purity is desired,as for pharmaceutical purposes, the suspension may be further'carbonated up to the isoelectric point to precipitate the calcium, carebeing taken not to 'pass such point. The magnesia suspension is thenseparated 'from the precipitate.

Particularly when the liquid-phase of the suspension is maintainedcontinuously ina state of unsaturation with v respect to Ca('OH)2 but onthe calcium hydroxide side of the true isoelectr i'c point, the chalk isprecipitated in the form'of dense, coarse particles which can easily beseparated by gravity, "while any magnesia. that is present remains insuspension. When the process is to be employed for the separation ofcalcium and magnesium, I prefer to keep the concentration of the liquidphase quite low so as toinsure against enrichment by the incomingmaterial to the saturated solid calcium hydroxide contain- 2,198,640 ingcondition, and especially to the secondary.

per liter, the preferred concentration in accord-' ance with theinvention, is maintained at approximately 0.15 gram per liter.

It should be noted that in my continuous process the calcium carbonateparticles remain, on the average, for a considerable timeinthe 'reactionliquid under carbonate precipitating conditions which are the same asthose under which the particles themselves were precipitated. This timeperiod will be larger the greater the body of liquid in the carbonatorin comparison with the volume of lime milk introduced per unit of time.It is my opinion that under these condiv tions the calcium carbonateparticles grow torelatively large size and are of uniform'characterbecause of the uniform conditions of precipitation.

In certain cases it is. desirable to keep the.

concentration of the Ca(OH)2 at approximately the range 3-0, as when aprecipitate whose properties are characteristic of such range isdesired, or when the feed of C02 is subject to such large fluctuationsthat there is, danger that carbonation at the range FG will be carriedinto the CO2 range upon sudden increase in the rate of CO2 feed. In suchinstances, the control instrumentis set-to increase slightly the rate'ofThe concentra- I continuouslythrough the overflow 26. The chalk can beseparated'fromthe magnesia suspension by gravity or by centrifuge andeven by'filtering,

as the colloidal solution"v readily passes through a filter. After thecarbonation has continued for same calcium concentration as the incominglime milk and willcontain a quantity of Ca(O.H)2 approximately equal tothat at which thecarbonate was precipitated.

To obtain the magnesia in usable form, the colloidal suspension ispassed rapidly through'a "some time, theefliuent suspension will be ofthe further carbonator wherein the carbonation is continued until theresistance of the suspension is raised nearly to or slightly beyond theisoelec-' tric point. This subsequent treatment withcarbon dioxideclumps or coagulates the colloid and renders it susceptible tofiltration or to concentration in a second centrifuge.

Other reagents may be used to break up th colloidal suspension, such asaluminum sulfate, sodium.carbonate, caustic soda and other electrolytes,but their use introduces an impurity, and I accordingly prefer the useof carbon dioxide, the resistance of the suspension beingheld-preferably beyond butnear to the isoelectric point to avoid loss ofmagnesium by solution as the bicarbonate.

In practice I find it desirable to slake the dolomite lime to a limemilk containing about 6% of particles should be of such small size thatthey tion of Ca (OH)2 then lies in the range between saturation and theconcentration corresponding to the resistance at 0.

As can be seen from Fig. 2, the resistance is practically the same forconcentrations from H to B, so that it would be difiicult to maintainthe concentration at a fixed value or .limited range of values withinsuch range H-B by measurement of the resistance. In such case, theproper concentration may be maintained by means of periodic titration,assuggested above,

or in any other suitable manner.

The precipitated chalk produced in accordance,

crates of particles of sub-microscopic size. This latter type ofprecipitate is-also produced when the carbonation is conductedcontinuously on the CO2 side-of the isoelectric point.

While the temperature at which batch carbonation takes place is veryimportant, as it determines the character of the precipitated chalk,

the specific temperature of carbonation is of relatively minorimportance in'my continuous proc ess. In the carbonation of adolomiticlime suspension, the temperature of carbonation has, however, an effectupon the. character of the I supernatant magnesia suspension, continuouscarbonation, at. 40 C. generally giving a more hydrous MUCH): than"material made at 20? C.

When the material to be carbonated is a suspension of calcium hydroxide*and magnesia,

there is obtained a surprisingly stable colloidal suspension of magnesiacarryingdense particles the body ofv liquid in the mainder being calciumcarbonate, while the heavy setting mud will comprise 90 to 95% ofcalcium carbonate, the remainder being magnesium oxide. The separationmay be accomplished more rapidly by passing the suspension through acentrifugal machine, the eflluent liquid carrying ofi the magnesium,while-a heavy calcium carbonate sludge remains behind. Depending uponthe speed of the centrifuge and the time in the basket, the eilluent.extremely stable colloidal suspension'of magnesia may vary from to 99%M80. a The product is a magnesium oxide-hydroxide which will calcine tothe 'oxide at a low temprature, incontrast to the products obtained a byknown processes, such as the Patterson process, which are basicmagnesium carbonates and require a higher acalcination temperature andyield a flufiier and lighter product.

The magnesia obtained as above described. and having an MgO contentof ofhigher is well suited for .the manufacture of refractory material. Uponfurther purificatiom-a material'suitable for pharmaceutical use may beobtained. It will be obvious that my process may be employed to enrich adolomitic lime in MgO by partial-precipitation of the calcium.

If desired, a wetting agent or protective colloid may beadded to thereaction mixture to facili- 4 magnesia colloidal suspension. Theprotective colloid should, of course, not be disturbed by alkalineconditions and high Ca ion concentrations. Sulfonated higher alcoholsare suitable forthis purpose.

I claim:

1. The method 01, carbonating an aqueous suspension of calcium hydroxidewhich comprises charging said suspension and; carbon dioxidecontinuously into a reaction vessel into intimate contact with eachother and maintaining the I concentration of the Ca(OH) 2 in the liquidphase of the suspension at a value of the order of about 10 to 15% ofsaturation. 1

2. The method of carbonating calcium hydroxide which comprisesintroducing lime milk into a body of water until the concentration ofcalcium hydroxide has reached a value of the order of about 10 to 15% ofsaturation, then continuously introducing carbon dioxide simultaneouslywith milk of lime, regulating the relative feed oicarbon dioxide andmilk oi lime in such mannor that the aforesaid concentration is main-'tained, and withdrawing precipitated carbonate.

3. The method of carbonating lime which comprises charging a milk oflimesuspension containing dissolved and undissolved calcium hydro xide andcarbon dioxide continuously and simultaneously into a reaction vesselcontaining an unsaturated solution of calcium hydroxide, so

relating the feed of the reacting materials that" carbonate from saidsolution, continuously drawing oil the suspension of calcium carbonatein an unsaturated solution of calcium hydroxide, and then separating thecalcium carbonate from said unsaturated calcium hydroxide solution.

HORACE E. STUMP.

